The rapid expansion of the number of cellular radio telephones coupled with the desire to provide additional services has prompted the Telecommunications Industry Association (TIA) to propose a new standard for a U.S. Digital Cellular Network. This standard suggests an increase in system capacity over the current analog system through the use of digital modulation and speech coding techniques. Time division multiple access (TDMA) is used to split the current channel into user slots. The linear modulation technique to transmit the digital information within the channel is .pi./4 QPSK (quadrature phase shifted keying).
The use of .pi./4 QPSK linear modulation in the U.S. Digital Cellular system provides spectral efficiency allowing the use of 48.6 kbps channel data rates. .pi./4 QPSK transmits the data information by encoding consecutive pairs of bits into one of four phase angles (.+-..pi./4,.+-.3.pi./4) based upon gray encoding. These angles are then differentially encoded producing an 8 point constellation. Differential encoding makes it possible to detect this modulation through the use of either non-coherent or coherent techniques.
The U.S. Digital Cellular system will operate in the existing 800 MHz band. Radio propagation at these frequencies is characterized by time dispersion distortion. Time dispersion distortion of a received signal occurs when a transmitted signal is received via more than one propagation path each having a different path length. Measured received signals having time dispersion distortion typically have a strong first component and multiple components that are generally lower in amplitude for larger delays. Time dispersion distortion of the received signal is usually found in an environment where a large reflecting source, such as a mountain, is present. A mobile radio in this environment receives the signal from a fixed source transmitter and the delayed signal from the reflecting source. The time delay between the reception of the two signals results in time dispersion distortion. Time dispersion distortion is also known as delay spread distortion. At high data rates (48.6 kbps for example), the time dispersion distortion introduced in the received signal by the channel needs to be considered in the bit error rate (BER) performance evaluation of the various demodulation methods.
The TIA standards committee has recommended a two path equal ray channel model with up to a symbol time delay (41.6 microseconds) interval between the two rays (IS-54, EIA/TIA standard section 2.3.2.1.2). This model significantly departs from the published delay profiles (10 microseconds) for typical urban, suburban, and urban propagation environments. The amount of intersymbol interference (ISI) due to the time dispersion distortion of the channel determines the method employed to produce acceptable BER performance. Methods considered to provide acceptable BER performance for a received signal during time dispersion distortion use non-coherent and coherent detectors, and a coherent detector in combination with a decision feedback equalizer (DFE). The scheme selected directly impacts the complexity of the receiver and acceptable BER performance.
The first detection method considered was a noncoherent limiter-discriminator with 1-symbol integration. Limiter-discriminator detection is possible due to the fact that the information content of the .pi./4 QPSK signal is in the phase shifts and not in the amplitude. Limiter-discriminator detection is the easiest method of implementing a .pi./4 QPSK detector since it uses familiar FM receiver technology.
The second method considered was a non-coherent delay detector. Detection is accomplished by multiplying the desired signal by a delayed version of itself. The delay detector requires a linear receiver to properly detect .pi./4 QPSK. This adds complexity to the receiver compared to the limiter-discriminator detector.
The third method considered was a coherent detector. This detector is based upon an open-loop approach. The coherent carrier is generated by quadrupling the modulated signal which produces spectral lines at 1/2 the symbol rate. By multiplying the quadrupled signal by the 1/2 symbol clock a coherent carrier is generated. The carrier is bandlimited and its phase angle is divided by 4 to generate the true carrier. A 90.degree. phase ambiguity results due to the quadrupling and dividing process. The recovered carrier is then used to detect the baseband "I" and "Q" signals. The coherent detector is the most complex detector compared to the delay and limiter-discriminator since it requires a linear receiver and additional circuitry to extract the coherent carrier and to detect the incoming signal.
The fourth method considered was a maximum likelihood sequence estimation. While this method may be feasible to use, it was considered not practical because of the large amounts of processing time and space needed in a signal processor to carry out the distortion eliminating task.
The fifth method considered was a linear transversal equalizer (LTE). The LTE was found to be unstable since it takes an infinite number of coefficients to meet the TIA channel model specification for delay spread distortion in the received signal.
An adaptive decision feedback equalizer (DFE) provides a powerful means to reduce ISI produced by the time varying time dispersion channel which exhibit spectral null characteristics. The equalizer must operate adaptively to track the channel variations during a TDMA frame slot. Fast convergence algorithms are required to train and follow rapid channel variations. To obtain fast convergence, the family of more complex recursive least-squares (RLS) algorithms is used in order to update the (DFE) equalizer coefficients.
Bit error rate results have been investigated for the non-coherent and coherent detectors. Presently, problems exist which would not allow the limiter-discriminator or delay non-coherent detectors to meet the current TIA delay interval specification.
As stated above, the coherent detector is the most complex detector to produce, but is the most likely detector for which equalization methods may be used. Thus, there is a need for a coherent receiver detector using a DFE, operating at high data rates, that meets the TIA channel model specification for time dispersion distortion.